Electrolysis of Aqueous Solutions

Position of the ions in the electrochemical series,

For positive ions, the facility of discharge decrease in going from those least electropositive to those most electropositive. For example, if both copper and hydrogen ions are present in solution, it will be the copper ions which take electrons from the cathode to become copper atoms.

For negative ions, the ease of discharge decrease in going from those least electronegative to those most electronegative.

Concentration of the Ions in Solution

Irrespective of the position of the ions in the electrochemical series, there is a tendency to promote the discharge of the most concentrated ion present. For example, in concentrated sodium chloride solution (i.e. brine) , the two cations present are the chlorine ion and the hydroxyl ion. Although the hydroxyl ion is more easily oxidised than the chlorine ion, it is the chlorine ion which will be discharged because its concentration is much greater than that of the hydroxyl ion.

Nature of the Electrode

This is not as important as either of the other two factors, except in certain cases. For example in the electrolysis of molten sodium chloride using a mercury cathode, sodium ions are discharged in preference to hydrogen ions which are lower in the series.

Electrolysis of an Aqueous Copper Sulphate Solution using Copper Electrodes

The electrolysis of an aqueous solution of copper sulphate using copper electrodes (i.e. using active electrodes) results in transfer of copper metal from the anode to the cathode during electrolysis. The copper sulphate is ionised in aqueous solution.

		CuSO4   ==>   Cu(++)   +  SO4(-.-)	

The positively charged copper ions migrate to the cathode, where each gains two electrons to become copper atoms that are deposited on the cathode.

		Cu(++)   +   2e(-)   ==>   Cu	

At the anode, each copper atom loses two electrons to become copper ions, which go into solution.

		Cu   ==>   Cu(++)   +   2e(-)	

The sulphate ion does not take part in the reaction and the concentration of the copper sulphate in solution does not change. The reaction is completed when the anode is completely eaten away. This process is used in electroplating.

Electrolysis of an Aqueous Solution of Sodium Sulphate

Cathode Reaction : 
		4 H2O   +   4 e(-)   ==>   2 H2   +   4 OH(-)	

The overall cell reaction is : 6 H2O ==> 2 H2 + O2 +4 H(+) +4 OH(-)

If the reaction is carried out in a Hofmann Voltammeter, with some universal indicator in the solution, it will be noticed that around the cathode the solution becomes alkaline and around that anode the solution becomes acidic. This is explained as follows :

At the cathode :

Hydrogen ions are being removed from solution, thereby leaving an excess of hydroxyl ions which makes the solution alkaline, and

At the anode :

Hydroxyl ions are being removed, so leaving an excess of hydrogen ions which makes the solution acidic.

Electrolysis of a solution of dilute Sulphuric Acid

Sulphuric acid is a strong electrolyte is fully dissociated in aqueous solution.

		H2SO4   ==>   2 H(+)   +   SO4(2 -)	

Water is a weak electrolyte and is only slightly dissociated

		H2O   ==>   H(+)   +   OH(-)	

During electrolysis, the hydrogen ions migrates towards the cathode, and are discharged there (i.e. they gain an electron and are converted to hydrogen gas).

		2 H(+)   +   2 e(-)   ==>   H2

At the anode the concentration of hydroxyl ions is too low to maintain a reaction and the sulphate ions are not oxidized but remain on in solution at the end. Water molecules must be the species reacting at the anode.

		2 H2O   ==>   O2   +   4 H(+)   +   4 e(-)	

The overall reaction is

For every hydrogen ions discharged at the anode, another hydrogen ion is formed at the cathode. The net result is that the concentration of the sulphuric acid remains constant and this electrolysis consists of the decomposition of water with the overall reaction

		2H2O   ==>   2H2   +   O2